This is an exploratory/developmental (R21) application under the Cutting-Edge Basic Research Awards (CEBRA) program. The pathogenesis of drugs associated with abuse behavior, including nicotine and alcohol addictions, remains elusive in humans because studies of the human brain are limited to functional brain imaging and post-mortem analysis. These types of analyses make it difficult or impossible to prove hypotheses directly since the system usually cannot be manipulated or sufficiently controlled. A large number of genetic variants have been identified to be risk factors for addictive behavior in human, however, little is known about how these genetic variations impact the development of addictive behavior in humans. Recent advances in stem cell biology allow construction of induced pluripotent stem cells (iPSC) from adult cells derived from addicted individuals carrying identified genetic variants and provide possibilities for developing cell-based models of addiction. Addictive behavior in human is not only related to cellular level modifications in a specific cell type in the brain but it also affects neuronal function such as synaptic plasticity at the neurocircuitry level. However, there are currently no such in vitro neurocircuitry models that have been established using human neurons. We hypothesize that neurons derived from subjects with risk-associated genetic variants will desensitize reward circuit modulation in an in vitro mini-neurocircuitry model. By using a compartmentalized culturing system, we propose to construct a mini-neurocircuitry model mimicking mesolimbic nucleus accumbens (NAc) neurons and their synaptic inputs. Cellular and synaptic phenotypes of neurons derived from addictive patients will be investigated under the context of neurocircuitry and compared with wild-type controls. This mini-neurocircuitry model will be essential to identify mechanisms underlying risk-associated gene variants and addictive behavior. It will also serve to develop and screen novel interventions for drug abuse therapies.